Few products are sold by their manufacturer without some type of testing being conducted. Such testing may be as simple as manually ascertaining whether certain parts are securely affixed—or as complex as “stress testing.” In stress testing (or “stress screening” as it is sometimes called), products exhibiting “infant mortality” fail outright during the test. Or as the result of such testing, a product may display evidence of early failure in the operating environment.
One of the most common methods of stress testing involves testing a product by subjecting it to vibrations of the type, which might be encountered in actual product use. For example, U.S. Pat. No. 2,438,756 (Larsen) explains that the apparatus described therein is used to vibration-test electrical apparatus for airplanes, ships and the like. The unit described in U.S. Pat. No. 3,748,896 (Barrows) is said to be used for testing parts of a motor vehicle. And vibration testing is often conducted in conjunction with testing using another regimen, e.g., temperature.
One type of vibration testing is known as repetitive shock testing. Such testing is generally accomplished by utilizing a testing apparatus consisting of a table frame that is vibrated by a number of vibrators which impart vibration through impacts occurring in each vibrator. These vibrators are generally pneumatically powered. During the testing process a uniform vibration response is desirable because it ensures that all components being tested are exposed to approximately equal vibration levels over the entire table frame.
Many different vibrator designs have been developed for use in vibratory testing systems. The primary focus of these designs to date have been to create a vibrator that imparts vibration onto a table frame and thus onto the object to be tested. These designs vary the physical design of the vibrator in order to create a vibrator that is capable of free running when connected to a supply of pressurized air. For example, U.S. Pat. Nos. 5,154,567 to Baker et al., 5,365,788 to Hobbs, and 5,493,944 to Felkins et al. all utilize varied channels and/or cut-outs on the piston within the chamber to create a vibrator that is capable of tree running once connected to an air source. In all of these designs the strength of the impacts and the frequency of the impacts generally increases as the pressure of the air supply is increased. In addition, some vibrator designs, such as '788 patent to Hobbs, allow the vibrator to randomly vary the strength of the impacts through the mechanical design of the piston itself.
The performance of a vibrator is usually shown as a power spectral density (PSD) which can be depicted as a chart showing g2/Hz over a determined number of different frequencies (Hz). FIGS. 17 and 18 show two such examples. FIG. 17 shows the PSD of a typical, impactor free running at 30 Hz. As can be seen, the chart shows numerous peaks at the harmonies of 30 Hz; this is commonly known as the “picket fencing” of the PSD. As understood by those of skill in the art, these peaks are not desirable since they represent frequencies at which the product is not properly tested. As shown in FIG. 18, by modulating the air pressure into that typical vibrator the peaks of the “picket fence” are reduced and widened.
Another measurement of performance of vibrators is the acceleration imparted by each impact. As described above, in many typical vibrators as die pressure of the air supply is increased the amplitude of the acceleration and frequency of the impacts increase together. The effect is seen in FIGS. 8-10 which show the strength and number of impacts overtime at high, medium aid low pressures respectively.